DNA helicase is an enzyme that unwinds double-stranded DNA to a single strand, and plays an important role in various processes associated with genetic information such as DNA replication, repair, transcription, translation and recombination. There are a variety of types of DNA helicases. DNA helicase showing homology with the RecQ helicase of Escherichia coli is referred to as “RecQ type helicase.”
Human genome includes 5 types of RecQ type helicases (RecQL1, WRN, RTS, BLM and RecQ5). Among these, mutations in the WRN, RTS and BLM genes cause genome instability diseases, namely Werner's syndrome, Rothmund-Thomson syndrome and Bloom's syndrome, respectively. On the other hand, involvement of RecQL1 and RecQ5 in diseases has not been reported.
RecQL1 helicase (also referred to as RecQ1 or RecQL) is considered to unwind the higher-order structure of DNA frequently seen during DNA replication, which is referred to as a holiday structure, and to promote DNA replication. In addition, RecQL1 is considered to form a complex with an MSH2/6 protein associated with mismatch repair, and to carry out the mismatch repair during genome replication. Moreover, it is known that RecQL1 is highly expressed in cancer cells and actively proliferating cells, but that the expression level thereof is low in cells at resting phase. The outline thereof is described in Futami et al. (2010), Pharmaceutical and Medical Device Regulatory Science PMDRS, 41(1): 19-26.
We earlier reported that mitotic catastrophe and mitotic cell death are induced to various cancer cells by a reduction in the expression level of RecQL1 helicase due to siRNA (WO 2004/100990, WO 2006/054625, JP Patent Publication No. 2012-219085 A, K. Futami et al. (2008), Cancer Sci., 99(1): 71-80, K. Futami et al. (2008), Cancer Sci., 99(6): 1227-1236 and K. Futami et al. (2010), Int. J. Mol. Med., 25: 537-545). It is believed that this is caused by the phenomenon that DNA damage generated as a result of DNA replication cannot repaired due to a reduction in RecQL1 helicase, and thereby the damaged cells directly proceed to the division cycle. We also reported that such siRNA that acts on RecQL1 shows antitumor activity in cancer-bearing model animals (WO '990, K. Futami et al. (2008), Cancer Sci., 99(6): 1227-1236 and K. Futami et al. (2010), Int. J. Mol. Med., 25: 537-545).
A method of inhibiting gene expression by siRNA is a widely applied research method. Since siRNA can be designed to target a specific sequence, it has high specificity and, thus, application of siRNA to pharmaceutical agents will be expected. However, since RNA is easily degraded by nuclease, it is problematic in that it is difficult for the RNA to exhibit a desired function when it is administered as a pharmaceutical agent to a living body. To overcome such a problem, it has been attempted to artificially chemically-modify a polynucleotide chain that constitutes siRNA by methylation, fluorination and the like to enhance the stability of the siRNA. However, it is known that the RNAi activity of siRNA is generally reduced by such modification.
It could therefore be helpful to provide: modified siRNA inhibiting the expression of a RecQL1 helicase gene; an agent inhibiting RecQL1 gene expression and an agent that induces cell death, each comprising the modified siRNA; and a pharmaceutical composition that treats cancer comprising the modified siRNA.